The TST contained 2 tuberculin units (TU) of RT23 tuberculin with Tween 80 (0.2 ml of 1TU/0.1 ml) given intradermally on the left forearm volar surface by Mantoux technique. Validity measures such as inter-observer performance were done for TST placement or interpretation individually by two experienced nurse. A purified protein derivative-Siebert (PPD-S) dose of 5TU is roughly equivalent to the 2TU of RT23. 2TU is the International Union Against Tuberculosis and Lung Disease recommended dose for RT23 TST. TST was also performed on BCG-vaccinated health care workers . After 48–72 h of injection, the TST site should be evaluated for induration and a transverse measurement was recorded. Any readings made outside of the above time period were excluded from the study. All unvaccinated subjects had their blood drawn for the QFT-RD1 the same time with applying the first TST. A TST reaction size of ≧10 mm was interpreted as a ‘positive’ TST which guided any further medical evaluation. This is the traditional cutoff point used for RT23 tuberculin in Taiwan as well as in most studies worldwide. If initial TST diameter less than 10 mm, second TST was repeated within 1-5 weeks to observe any ‘boosting phenomenon’ was possible. Statistical analysis of skin test results for each population characteristic was done at this cut-off point, with the exception of one analysis specifically done at≧18 mm in BCG-vaccinated health care workers.